Asymmetrical screw head slots



Oct. 4, 1960 VAUGHN 2,954,719

ASYMMETRICAL SCREW HEAD SLOTS Fild 001;. 24, 1957 as v l I RUDOLPH B Z-3 5; IIWIIIIIII BY mafiAW 8 ATTORNEYS United Sates Patent ASYMNIETRICAL SCREW IEAD SLOTS Rudolph M. Vaughn, 2008 N. Westwood,

Santa Ana, Calif.

Filed Oct. 24, 1957, Ser. No. 692,132

1 Claim. (CI. 85-45) This invention relates generally to slotted screw heads Patented Oct. 4, 1960 when the driver is torqued in an opposite direction. In one embodiment, the driving surfaces engaged when and more particularly to an improved high torque slot set forth in said patent and need not be repeated here.

There are many instances, however, where over-torquing of a screw can so strain the members being fastened together or even portions of the screw shank itself that an inherent weakness in the fastening may be introduced. In such applications of the high torque screw, it is, therefore, necessary that the operator the maximum torque applied to the screw in tightening the same. To this end, it is common practice to provide a torque limit wrench in which only a given maximum torque can be applied by the wrench. Alternatively, the operator can in many instances judge by the torque manually applied to the wrench when the proper pre-loading has been effected and at which point further torque might result in undesirable yielding of internal portions of the fastening.

While the torque limit wrench or even careful manual control by an operator are relatively effective in insuring that a fastening has been properly secured, there are still many instances particularly when thousands of such high torque screws are employed such as in aircraft construction, where a certain percentage thereof may be torqued beyond a safe value. At present, there is no convenient manner of determining whether a screw has been torqued beyond a safe value by simple visual inspection. To be absolutely certain of the safety of the fastening, it is necessary for an inspector [to go over each of the fastenings with a torque wrench and by feel determine whether proper torque has been applied or whether the device has been over or under-torqued. This inspection operation is not only time consuming but in itself is subject to error and is costly from a labor standpoint.

Bearing the above in mind, it is a primary object of the present invention to provide an improved high torque slotted screw head in which the maximum torque which may be applied to the slot in predetermined by th physical construction of the slot itself.

Another object is to provide a slotted screw head which will indicate by simple observance whether or not it has been over-torqued when employed for a fastening-device.

A more general object of the invention is to provide an asymmetrical screw head slot in which more torque may be applied in one direction than in the other.

These and many other objects and advantages of the present invention are attained, briefly, by providmg the driver is turned in one direction are conventional flat plane surfaces which are not undercut such that a torque limit exists at that point at which the driver is camrned out of the slot, while the surfaces engaged when thedriver is torqued in the other direction are of the undercut type in which full surface engagement is bad with the driver. In a second embodiment, the driving surf-aces engaged when the driver is torqued in one direction are undercut non-planar curved surfaces of revolution of less surface area and thus less engaging area than the surface portions engaged by the driver when torqued in the other direction whereby again the maximum torque which may be applied in one direction is limited by the tendency for the driver to cam out of the slot when a predetermined torque has been exceeded.

A better understanding of the various asymmetrical screw head slots of this invention will be hed by referring to preferred embodiments thereof as illustrated in the accompanying drawings, in which:

Figure 1 is a schematic exploded perspective view illustrating certain elements involved in the formation of the asymmetrical screw slots of this invention;

Figure 2 is another diagrammatic view useful in explaining the method of making the asymmetrical slots of the invention;

Figure 3 is an enlarged perspective view of a slotted screw head of the type disclosed in the aforementioned United States patent;

Figure 4 is a perspective view similar to Figure 3 but showing a modified type of asymmetrical slot in accordance with a first embodiment of the present invention;

Figure 5 is a plan view of the screw head and slot illustrated in Figure 4;

Figure 6 is another enlarged perspective view of a second embodiment of an asymmetrical slot in accordance with the present invention;

Figure 7 is a plan view of the slot illustrated in Figure 6; and

Figure 8 a cross section of the screw head taken in the direction of the arrows 8-8 of Figure 6.

Referring first to Figure 1, there is illustrated a screw head 10 at the upper end of a screw shaft 11 provided with a high torque recess or slot 12. The slot 12 may be milled out of the top of the head 10 by means of a cutter wheel shown at 13 provided with cutting teeth about its periphery as indicated at 14. The actual cutting wheel employed would be provided with cutting teeth shaped such as the two teeth 14 about its complete periphery cut from the blank 13. In order to clarify the explanation of the generated surfaces constituting the. sides of the slot, however, the cutting wheel is illustrated in full lines without all of the teeth.

The method of milling the slot 12 in the screw head to by means of the cutter wheel 13 is fully set forth in my US. Patent Number 2,745,120 issued May 15, 1956 and entitled Method of Milling an Undercut Slot in 2. Screw Head, but is reviewed briefly here to set a foundation for a proper understanding of the improved asymmetrical slots of the instant invention.

Referring particularly to Figure 2, the cutting wheel 13 is illustrated in dotted lines and in the milling out of the slot 12 the wheel is lowered onto the top surfaces of the screw head 10 and initially a transverse slot is cut to a certain depth. The bottomof this slot will thus be arcuate with a radius of curvature corresponding to the radius of the cutting wheel 13 and terminating on the longitudinal axis AA of the screw shaft 11 as shown in Figure 1. To provide the desired undercut portions, after the slot has been 'cut to a desired depth, with the cutting wheel still rotating rapidly, it is caused to swivel or oscillate through a small angle C as indicated in Figure 2 on either side of the transverse axis of the slot. The periphery of the wheel itself is defined by diverging peripheral side walls such that the desired undercutting as indicated by the dotted lines in Figure 2 is effected.

Referring once again to Figure 1, it will be noted that these peripheral diverting side wall portions of the cutter wheel 13 constitute surfaces of revolution as indicated by the projected surface of revolution 15 in Figure 1. Consequently, upon swiveling movement of the cutting wheel asindicated in Figure 2 the undercut portions of the side walls of the slot 12 willnecessarily constitute non planar curved surface portions of revolution. The generating axis for these surface portions is illustrated at BB in Figure 1 and in the particular embodiment of the cutter blank disclosed, these surface portions are conical surface portions, the apex of the cone of the projected surface 15 being indicated at P in Figure 1.

Referring specifically to the enlarged perspective view of the slot 12 illustrated in Figure 3, it will be noted that as a consequence of the foregoing cutting operations, the slot comprises opposing plane side wall portions 16 and 17 in its central region which'are parallel to each other and separated by a distance equal to the width of the slot at its central region, which in turn corresponds to the outermost peripheral width of the cutting Wheel. Because of the particular diverging peripheral side wall construction of the cutting wheel, there will be left the referred to plane parallel opposing surfaces 16 and 17 in the central region of the slot and these plane surfaces will be substantially triangular in shape as indicated. The swiveling movement of the wheel results in the provision of undercut non-planar curved surface portions of revolution diverging from the central region towards the periphery of the head such as indicated at 18, 19, 20 and 21.

If a screw driver is now provided with a driving end of arcuate configuration and diverging side walls constituting surface portions of revolution such as would result by taking a sector from the cutting blank 13 of Figure 1, driving of the screw head 10 by such a driver would result in'full surface engagement over the entire areas of the undercut diverging portions 18 and 19, when the screw head 10 is torqued in a clockwise direction or 20 and 21 when the screw head is torqued in a counter clockwise direction. As a consequence of this full surface engagement and of the undercut feature, very high torques may be applied to the screw 'head 10 without the driver camming out of the slot or without a yielding of' the metal in the head taking place.

As mentioned heretofore, there are many instances, however, in which it is important that the screw not be over-torqued or loaded beyond a given limit when securing members together and in accordance with a first embodiment of the present invention, the screw head slot is designed to limit the maximum torque that can be applied. This first modified type of slot is illustrated in the screw head 22 of Figure 4 wherein the slot comprises opposing parallel plane side walls 23 and 24 and diverging portions extending in diagonally opposite directions from the central region constituting undercut non-planar curved surface portions of revolution 25 and 26. It will be immediately evident from Figure 4 that. the diverging portions 25 and 26 are asymmetrical to their respective opposite surface portions 23 and 24.

To form the asymmetrical screw slot illustrated in Figure 4 the cutter wheel 13 of Figure 2 is simply swiveled in one direction from the central transverse axis through the angle C and brought back to a center position without any swiveling to the other side of the transverse axis. As a consequence, the driving portions of the slot when the screw is to be tightened or turned in a clockwise direction constitute the two plane surfaces 23 and 24 and these surfaces, not being undercut, will be limited in the torque which they can accommodate. The bottom of the slot 27 is arcuate. 'Thus, by proper design of the material employed in the head 22 and the depth to which the slot therein is cut, the areas of the plane surfaces 23 and 24 may be of a value such that applying torque beyond a predetermined limit will simply result in a yielding of the metal and cammin out of the driver from these side wall portions. On the other hand, a much larger torque may be applied in a counter clockwise direction on the screw head 22 as a consequence of the undercut design of the side walls 25 and 26 engaged by the driver when the latter is turned in a counter clockwise direction.

.The dis-symmetry between the diverging surface portions 25 and 26 with respect to their opposite surfaces 23 and 24 will be evident from the plan view of Figure 5 wherein it will be noted that the plane surfaces 23 and 24 are similar to the conventional side walls of a slotted screw head and are thus subject to camming out of the driver when torqued in a clockwise direction more readily than in the case when the driver engages the diverging curved surfaces 25 and 26. v

Figure 6 illustrates a second embodiment of applicants invention in which an asymmetrical slot is provided for the same purposes of limiting the maximum torque which may be applied to the screw in a clockwise direction. This slot includes opposed parallel plane portions 29 and 30 in its central region and undercut diverging surface portions 31, 32, '33, and 34. The slot in the screw head 28 of Figure 6 is cut by swiveling the cutting wheel 13 to the left of the transverse axis of the slot as viewed in Figure 2 in a normal manner. However, upon swiveling movement of the cutter wheel 13 to the right of the transverse axis in a clockwise direction,.the entire cutter wheel is raised slightly as indicated by the arrow D in Figure 1. As a consequence, the generating axis for the non-planar curved surfaces of revolution 31 and 32 is spaced a distance d above the normal rotation axis B-B of the cutter wheel as indicated in Figure 1. Consequently, these diverging surface portions are of less area than the undercut non-planar surfaces of revolution 33 and 34 of Figure 6. Preferably, the distance d is less than the slot depth at its central region.

The asymmetrical slot of Figure 6 may be better appreciated by reference to the plan view of Figure 7 and cross sectional view of Figure 8 wherein it will be noted that the undercut portions 31 and 32 are of a lesser extent than the undercut portions 33 and 34. As a further consequence of the raising of the cutterwheel slightly when it is rotated in the one direction to form the undercut surface portions 31 and 32, the bottom ofthe slot is also raised up as indicated at 35 in the cross sectional view of Figure 8. In addition, the end points of the diverging surface portions 31 and 32 where they meet the surface of the screw head are radially spaced from the longitudinal axis of the screw a distance less than the radial distance of the termination points for the .surface portions 33 and 34 as will be clear from the plan view of Figure 7. 7

From the construction illustrated and described, it will be evident that when the screw driver is torqued in a clockwise direction, the engaging surfaces are of considerably smaller area than when torqued in a counter clockwise direction with the desired result that the maximum torque which may be applied in tightening the screw is limited by the tendency for the driver to cam out of the slot as a consequenceof yielding of the slot edges defining the reduced area non-planar curved engaging surfaces.

It will be apparent from the foregoing descriptionthat if asymmetrical slotted screw heads such as illustrated in either Figure 4 or 6 are used, evidence of an attempt to over-torque or tighten the same beyond the desired limit will be immediately evident by simply inspecting the slot structure itself to see if it has been damaged as a consequence of camming of the driver out of the slot. On the other hand, when the driver is turned in an opposite direction, the desired full surface contact takes place over a greater area because of the larger area of the diverging surface portions 25 and 26 in the case of Figure 4 and 33 and 34 in the case of Figure 6 whereby a larger torque greater than the maximum than can be applied in a clockwise direction will enable ready removal of the screw.

It will also be immediately appreciated from the foregoing that the asymmetry of the slots may be designed in a manner reversed to that described such that high torque may be applied in tightening the screws whereas the available torque for loosening the same is limited by the slot structure. In other words, in the case of the embodiment of Figure 4, by rotating the cutter wheel to the right as indicated in dotted lines in Figure 2 and returning it to the center line without passing to the left, the plane surface portions 23 and 24 would be undercut in a manner similar to the undercut surface portions 18 and 19 of Figure 3 while the diverging opposite surfaces 25 and 26 would be simply plane surfaces similar to the surfaces 23 and 24.

In the case of the asymmetrical slot of Figure 6, if the cutting wheel 13 were raised when it was being swiveled to the left as viewed in Figure 2 rather than when swiveled to the right, the diverging portions 33 and 34 would be reversed with respect to the reduced area diverging portions 31 and 32 so that again a very high torque could be applied in tightening the screw head whereas a limited torque only could be applied for untightening the same. These latter embodiments would be useful wherein it is desired to provide a permanent fastening which is substantially tamper-proof. In other words, because a much higher torque could be applied in tightening the screw as compared to unthreading or loosening the same, once it was tightened it would not be possible to loosen the screw since the necessary unthreading torque could not be accommodated by the slot walls engaging the driver when torqued in the counter clockwise direction.

Various further modifications within the scope and spirit of the present invention will readily occur to those skilled in the art. The invention is, therefore, not to be thought of as limited to the specific embodiments disclosed for illustrative purposes.

What is claimed is:

- A screw head having a longitudinal screw axis and comprising a slot extending along a transverse axis perpendicular to the longitudinal axis across the top of the head, said slot having opposite walls with central portions parallel to said axis and an arcuate bottom between and intersecting said walls, a portion of the arcuate bottom having a center of curvature above the head at said longitudinal axis, two pairs of undercut driver-engaging surfaces one of each pair of which is in opposite wall portions of the slot on diametrically opposite sides of the longitudinal axis and the other pair of which is in the other opposite wall portions of the slot on diametrically opposite sides of the longitudinal axis, the upper edge of each driver-engaging surface lying at the surface of the head and the lower edge of each driverengaging surface of one pair lying at and intersecting the arcuate bottom portion to define an arc of a circle, each lower edge of the other pair forming the arc of a circle equal in diameter to that of the said one pair, said bottom having a first curved portion defined by a boundary line on each side of the slot, said line being defined by the lower edge of a surface of said one pair and the intersection of an imaginary plane with said bottom, said imaginary plane coinciding with one of said opposing parallel walls which is adjacent said surface of said one pair, said bottom having two other curved portions each of which extends upwardly from said first curved portion to the lower edge of a surface of said other pair, the upper edge of each driver-engaging surface overlying its respective lower edge, whereby the width of the slot measured perpendicular to the transverse axis at the driver-engaging surfaces is greater at the bottom than at the top of the slot, said driver-engaging surfaces each having substantially the shape of a portion of a cone, each cone having the same shape and each having a central axis above the head which passes through and is substantially perpendicular to the longitudinal axis, the central axis of the cones that form the surfaces of said other pair being spaced a given distance above the point of the central axes of the cones forming the surfaces of said one pair a distance less than the depth of the slot at the screw axis, the surfaces of said other pair extending radially outwardly from the screw axis to a less extent than the surfaces of said one pair extend, the upper edges of the driver-engaging surfaces diverging from the transverse axis in the direction of the periphery of the screw head.

References Cited in the file of this patent UNITED STATES PATENTS 131,843 Brooks Oct. 1, 1872 1,956,963 Salmen May 1, 1934 2,304,704 OLeary Dec. 8, 1942 2,677,985 Vaughn May 11, 1954 2,684,094 Liss July 20, 1954 2,792,039 Wing et a1. May 14, 1957 

